1. Field of the Invention
The present invention relates to catalysts for petroleum refining, and particularly to an isomerization catalyst for converting straight chain hydrocarbons, particularly C7 and higher paraffins, to branched hydrocarbons in order to improve the octane rating of the feedstock.
2. Description of the Related Art
Ever since the advent of the internal combustion engine, gasoline and diesel fuel have been valuable commodities. Gasoline is a blend of hydrocarbons having a boiling point between 30-200° C. that is produced by distillation of crude oil. However, straight-run gasoline is a poor fuel because of the phenomenon of engine knock, which occurs when a portion of the air-fuel mixture in the combustion chamber ignites prematurely. Premature detonation experienced during engine knock causes engine wear from vibration, and severe knock may damage the engine.
It is known that engine knock is more severe with straight-chain hydrocarbons. Octane rating is improved with more branching in the hydrocarbons. One measure of the antiknock properties of gasoline is the octane number (diesel fuel uses a similar rating called the cetane number). There are two octane numbers of interest: the research octane number (RON) determined by testing that simulates driving under mild, cruising conditions; and the motor octane number (MON) that is determined by testing that simulates more severe conditions under load or at high speeds. In the U.S., the octane rating at the gas pump is the average of the RON and the MON. Heptane is assigned an octane rating of 0, while 2,2,4-trimethylpentane (isooctane) is assigned an octane number of 100. An engine performs best when the gasoline octane rating is matched to the compression ratio of the engine.
Most gasoline refining uses some form of catalytic cracking of C11-C14 kerosene to light hydrocarbons in the C3-C5 range, followed by catalytic reformation of the light hydrocarbons to highly branched C7-C10 hydrocarbons. Formerly, lead compounds, particularly tetraethyl lead, were used as gasoline additives to reduce knock. More recently, various oxygenated hydrocarbons, such as methyl test-butyl ether (MTBE) and ethanol have been used to increase the octane number and oxygenate gasoline for more complete combustion. However, the use of such additives, particularly lead and MTBE, has become more restricted and, in some instances, legally banned due to environmental and health concerns. In addition, refiners must limit the amount of aromatics and oxygenated compounds due to concerns about toxic gas emissions.
With decreasing demand for benzene and related light aromatic hydrocarbons and increasing demand for gasoline in industrial and developing nations, many refiners have shown renewed interest in isomerization of C5-C10 hydrocarbons to branched hydrocarbons for use in gasoline. Several effective catalysts are available for isomerization of C5-C6 feedstocks for this purpose, and many refiners include such isomerization units in their refineries. However, C7-C10 hydrocarbons have proven more difficult to isomerize efficiently due to cracking reactions and to excessive production of aromatics and oxygenated compounds as byproducts, which are difficult or expensive to separate, making the process uneconomical.
Thus, an isomerization catalyst solving the aforementioned problems is desired.